The US government ordered a clutch of security reviews on Tuesday as investigators attempted to find out why a troubled former reservist walked on to the oldest military installation in the United States with at least one weapon, opened fire and killed 12 people before he was shot dead.

A senior Pentagon official said the defense secretary, Chuck Hagel, intended to order a review of physical security and access at all military installations across the world. Military leaders were being consulted over the parameters of the review, which should be formally announced on Wednesday, the official said.

The White House separately announced a review of security procedures for private contractors after it was revealed that Aaron Alexis obtained clearance to work on the base despite having been arrested twice in the past for gun-related offences.

Jay Carney, the White House spokesman, said it would be conducted by the Office of Management and Budget and examine "standards for contractors and employees across federal agencies".

A third inquiry was announced by the navy secretary, Ray Mabus, who ordered a "rapid review" of security at navy and marine corps installations.

The FBI revealed on that Alexis was armed with just a shotgun on Monday morning when, acting alone, he entered the military compound using a valid security pass and began firing at other civilian contractors.

Valerie Parlave, head of the FBI's field office in Washington, dismissed reports Alexis used an AR-15 semi automatic rifle – the weapon used in other recent mass shootings in the US. Instead she said Alexs used only the shotgun, bought in nearby Virginia, when he entered building 197. "We also believe Mr Alexis may have gained access to a handgun once inside the facility and after he began shooting," she said.

The FBI is still seeking to determine his motive and Parlave would not comment on reports that Alexis was receiving treatment for mental problems. "We continue to look into Mr Alexis's past, including his medical and criminal histories," she said.

Police named the the 12 victims of the attack on Tuesday. All were civilian employees aged between 46 and 73. It was unclear whether Alexis, 34, had deliberately avoided shooting military personnel.

The metropolitan police department in Washington said Alexis was believed to have shot other employees at the compound from the lobby of building 197 to the third and fourth floors. He had several firefights with police before being killed by armed officers. Police chief Cathy Lanier said the succession of firefights lasted between 30 minutes and an hour before he was shot dead.

The company that employed Alexis said on Tuesday that his latest background check revealed only a minor traffic violation. The Experts, a defence contractor, said the check on Alexis was carried out three months ago. The company said that had it known of his two arrests for gun-related offences, it would not have employed him.

A Virginia gun store and range also said it ran a background check on Alexis before selling him a gun and ammunition on Sunday. Sharpshooters Small Arms Range said that he passed the check.

In Washington, Hagel joined other military officials to lay a wreath at the navy memorial to honour those who died. The wreath was placed next to the statue of the Lone Sailor, to represent "all people who have ever served, are serving now, or are yet to serve in the United States navy".

Chuck Hagel lays a wreath at the US navy memorial. Photograph: Keith Lane/Corbis

There was still no clear motive for the killing spree, although a picture was emerging of a disturbed individual prone to sometimes angry outbursts, who may have been undergoing treatment for mental illness.

Although he was never convicted of a crime, Alexis had a number of encounters with police, two of which involved firearms, and had a history of disorderly conduct before he was granted an honourable discharge from the navy two years ago.

Police in Seattle said Alexis had been arrested in 2004 after shooting out the tyres of a car belonging to a worker on a construction site near his home. Alexis was said to have acted in an "anger-fuelled blackout", furious about where the car was parked.

Authorities in Fort Worth, Texas, said Alexis was arrested in 2010 when he was a navy reservist there, after an upstairs neighbour complained he had shot through the ceiling of her home. The police report said Alexis had complained about noise. He was not charged after police accepted his explanation that he had discharged the gun by accident.

The Associated Press reported that the Department of Veterans' Affairs had been involved in treating Alexis for a number of mental health issues since August. The police and the FBI, which is leading the investigation, declined to confirm or deny the report.

However, a number of friends and associates of Alexis suggested he had shown unusual behaviour. Gene Denby, a reporter at National Public Radio and a friend of Alexis's sister, recalled him apparently behaving strangely "six or seven" years ago.

Denby said that Alexis called his sister, incoherent, claiming "people were out to get him". Denby said: "She was unnerved, clearly unsettled by it."

There were growing questions over whether he should have been granted a security clearance. Thomas Hoshko, the chief executive of The Experts, which is headquartered in Florida and has a base in Alexandria, Virginia, said the US military granted him a "secret" clearance – providing him with a pass known as a common access card – and worked at six or more US military installations in July and August. "We had just recently re-hired him. Another background investigation was re-run and cleared through the defense security service in July 2013," he said.

The work in Washington, focusing on the navy and marine corps intranet, had only just begun. The FBI said Alexis had been staying in hotels in the Washington DC area since August 25, and checked into the Residence Inn hotel, near the facility in southwest Washington, on September 7.

On Tuesday, police were searching his room at the hotel, and appealing for any information that might provide insight into why Alexis opened fire on his co-workers.

It remained unclear whether the shootings would reignite the stalled debate in Congress over gun control. Calls for action came from an unexpected quarter, when the most senior clinician at a hospital that treated victims of the shootings broke away from giving updates about their medical conditions to make a powerful plea for Americans to eradicate the "evil in our society" that keeps her hospital so busy with shooting injuries.

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"I would like you to put my trauma center out of business," Janis Orlowski, the chief medical officer at Washington hospital center, told reporters in the aftermath of the killing. "I would like to not be an expert on gunshots. Let's get rid of this. This is not America."

In follow-up comments on Tuesday, Orlowski said her plea had been a spontaneous reaction after she observed how similar the shooting injuries had been to victims of routine gun crime in Washington.

"I have to say it was direct from my heart," she said in a series of television interviews. "I must have poked the underbelly, because I have gotten quite a few comments and phone calls and emails and tweets about it."

Weaknesses in the military's system of background checks were further highlighted on Tuesday by the publication of a damning report into a separate programme used for lower-level entry permits.

The report from the defense department's inspector general found that cost-cutting and bureaucratic bungles had resulted in a system that failed to pick up 52 "convicted felons [who] received routine unauthorized installation access, placing military personnel, attendants, civilians in installations at an increased security risk." Nine out of ten installations allowed contractors temporary access pending the results of background checks.

The Navy's Commercial Access Control System, under fire in the report, was designed to speed up routine access for contractors but Aaron Alexis went through a higher level of security clearance known as the Common Access Card.

"There is no indication that the findings in the report could have prevented, in any way, the tragic shooting at the Navy Yard," said rear admiral John Kirby, chief of information for the Navy. "The system they examined is not the system through which Mr Alexis gained access. Even if we had followed every recommendation in the IG report, we could not have prevented Monday's tragic events."